The present invention relates to an image producing method and X-ray CT (Computed Tomography) apparatus, and more particularly, to an image producing method and X-ray CT apparatus by which, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
FIG. 16 is a flow chart showing a prior art image producing method for producing an image based on data obtained by a helical scan employing a multi-row detector having more than one detector row.
In Step S1, preprocessing such as sensitivity correction is applied to the data.
In Step S3, multi-slice/helical interpolation processing is applied for calculating interpolated data from proximate data in an image reconstruction plane.
In Step S4, backprojection processing is applied to the interpolated data to produce an image.
FIG. 17 is a schematic diagram showing exemplary multi-slice/helical interpolation processing using a twin detector.
FIG. 17 expresses the twin detector as viewed from an X-ray tube at a view angle xcfx80/2.
The interpolated data D(xcfx80/2, i) at the view angle xcfx80/2 for a channel i is calculated by linear interpolation from data d1(xcfx80/2, i) at the view angle xcfx80/2 for the channel i in a first detector row (j=1) and data d2(xcfx80/2, i) at the view angle xcfx80/2 for the channel i in a second detector row (j=2).
The linear interpolation is used because the data d1(xcfx80/2, i) and data d2(xcfx80/2, i) lie on a line along the subject""s body axis, assuming that the CT value varies linearly in the direction of the subject""s body axis.
Since the scan plane is not tilted in FIG. 17, the position h1 of the data d1(xcfx80/2, i) of the first detector row (j=1) relative to the axis of translation is equal to the position h2 of the data d2(xcfx80/2, i) of the second detector row (j=2) relative to the axis of translation. That is, both the data d1(xcfx80/2, i) and d2(xcfx80/2, i) lie on a line along the subject""s body axis, which satisfies the condition required for linear interpolation.
When the scan plane is tilted, however, the position h1 of the data d1(xcfx80/2, i) relative to the axis of translation and the position h2 of the data d2(xcfx80/2, i) relative to the axis of translation become unequal, as shown in FIG. 18. That is, the data d1(xcfx80/2, i) and the data d2(xcfx80/2, i) no longer lie on a line along the subject""s body axis. As a result, the condition required for linear interpolation is not satisfied, leading to artifacts.
It is therefore an object of the present invention is to provide an image producing method and X-ray CT apparatus by which, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
In accordance with a first aspect, the present invention provides an image producing method for producing an image based on data collected by a helical scan employing a multi-row detector having more than one detector row with a scan plane tilted, characterized in comprising the step of: applying to the data tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to an axis of translation due to the tilt of the scan plane.
In the image producing method of the first aspect, tilt correcting processing is newly introduced for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
In accordance with a second aspect, the present invention provides an image producing method characterized in comprising the steps of: applying preprocessing such as sensitivity correction to data collected by a helical scan employing a multi-row detector having more than one detector row with a scan plane tilted; next applying tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to an axis of translation due to the tilt of the scan plane; applying multi-slice/helical interpolation processing for calculating interpolated data from proximate data in an image reconstruction plane; and applying backprojection processing to the interpolated data to produce an image.
In the image producing method of the second aspect, the tilt correcting processing is applied before the multi-slice/helical interpolation processing for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed using the same multi-slice/helical interpolation processing as conventionally used.
In accordance with a third aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that said tilt correcting processing includes data position shifting processing for shifting the positions of data arranged in a two-dimensional array along a channel index axis and a view index axis so that the view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane is canceled out; data extracting processing for extracting a range of data in which data are completely present for all the views in the view direction from the shifted data array; dummy data appending processing for appending the extracted data with dummy data to adjust the data range; and data transforming processing for transforming the data into data enabling alignment of the channel positions through all views.
In the image producing method of the third aspect, the positions of data are first shifted so that the view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of a scan plane is canceled out. However, the amount of data position shift is not limited to an integer multiple of the channel-to-channel distance. Accordingly, in the shifted data array, there occur concave and convex portions at the opposite ends of the channel index axis, and hence, a portion along the view direction in which data are completely present for all the views and a portion along the view direction that partially lacks data. The portion along the view direction in which data are completely present for all the views can be used as is, and a range of the data corresponding to the portion is extracted. On the other hand, using the portion along the view direction that partially lacks data may cause artifacts, and the portion is discarded. Missing data corresponding to the discarded portion are made up for with dummy data to adjust the data range. Since, as pointed out above, the amount of data position shift is not limited to an integer multiple of the channel-to-channel distance, the data positions in the resulting data array do not match the channel positions for all the views. Therefore, the data are transformed into data enabling alignment of the channel positions through all views by, for example, interpolation. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed using the same multi-slice/helical interpolation processing as conventionally used.
In accordance with a fourth aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that: said data position shifting processing shifts the positions of parallelized data of the channels in a j-th (j is the detector row index and 1xe2x89xa6jxe2x89xa6J) detector row by:
jxe2x80x94deltxe2x80x94iso=Ljxc2x7tan xcex8xc2x7sin{2xcfx80(pvnxe2x88x921)/VWN}
in the channel direction, where pvn is the view index and 1xe2x89xa6pvnxe2x89xa6VWN, rotation is made substantially for 2xcfx80 for all views, the tilt angle is represented as xcex8, and a distance from an intersection of the axis of translation and an axis of rotation to a scan plane corresponding to the j-th detector row is represented as Lj.
The amount of data position shift in the data position shifting processing varies with the conditions of a helical scan (e.g., the helical pitch), and the amount of shift shown in the image producing method of the fourth aspect represents an example.
In accordance with a fifth aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that: said data extracting processing extracts data from a (Roundup{Ljxc2x7tan xcex8/DMM}+1+j_delt_iso )-th channel to a (Ixe2x88x92Roundup{Ljxc2x7tan xcex8/DMM}xe2x88x921+j_delt_iso )-th channel in the j-th detector row for a pvn-th view, where DMM is the channel-to-channel distance, and Roundup{ } is a roundup function.
The range of data extracted in the data extracting processing varies with the conditions of a helical scan (e.g., the helical pitch), and the range shown in the image producing method of the fifth aspect represents an example.
In accordance with a sixth aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that said tilt correcting processing includes data extracting processing for extracting data from a (Roundup{Lj* tan xcex8/DMM}+1+j_delt_iso)-th channel to a (Ixe2x80x94Roundup {Lj* tan xcex8/DMM}xe2x88x921+j_delt_iso)-th channel in a j-th (j is the detector row index and 1xe2x89xa6jxe2x89xa6J) detector row for a pvn-th view, where pvn is the view index and 1xe2x89xa6pvnxe2x89xa6VWN, rotation is made substantially for 2xcfx80 for all views, the tilt angle is represented as xcex8, a distance from an intersection of the axis of translation and the axis of rotation to a scan plane corresponding to the j-th detector row is represented as Lj,DMM is the channel-to-channel distance, Roundup{ } is a roundup function, and
j_delt_iso=Lj* tan xcex8* sin{2xcfx80(pvnxe2x88x921)/VWN};
dummy data appending processing for appending the extracted data with dummy data to adjust the data range; and data transforming processing for transforming the data into data enabling alignment of the channel positions through all views.
In the image producing method of the sixth aspect, while the processing proceeds by sequentially executing the data extracting processing, dummy data appending processing and data transforming processing in this order without data position shifting processing, the same result as that by the image producing method of the fifth aspect can be obtained.
In accordance with a seventh aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that said dummy data are air data.
In the image producing method of the seventh aspect, air data (the CT value of the air) is used as the dummy data. This provides the best image.
In accordance with an eighth aspect, the present invention provides the image producing method as defined by any one of the inventions of the third aspect through the seventh aspect, characterized in that said data transforming processing is interpolation processing.
Although it is possible to copy the most proximate data and transform the data into data enabling alignment of the channel positions through all views, the image producing method of the eighth aspect uses interpolation processing for the transformation of data into data enabling alignment of the channel positions through all views. This provides the best image.
In accordance with a ninth aspect, the present invention provides an X-ray CT apparatus comprising an X-ray tube, a multi-row detector having more than one detector row opposed to said X-ray tube, translation control means for translating said X-ray tube and said multi-row detector along an axis of translation relative to a subject, rotation control means for rotating at least one of said X-ray tube and said multi-row detector around an axis of rotation, tilt control means for tilting the angle of a scan plane formed by said rotation relative to the axis of translation to an angle other than 90xc2x0, scan control means for collecting data by a helical scan employing said multi-row detector with the scan plane tilted, and image producing means for producing an image based on the collected data, characterized in that said X-ray CT apparatus further comprises: tilt correcting processing means for applying to the data tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane.
The X-ray CT apparatus of the ninth aspect can suitably implement the image producing method of the first aspect.
In accordance with a tenth aspect, the present invention provides an X-ray CT apparatus comprising an X-ray tube, a multi-row detector having more than one detector row opposed to said X-ray tube, translation control means for translating said X-ray tube and said multi-row detector along an axis of translation relative to a subject, rotation control means for rotating at least one of said X-ray tube and said multi-row detector around an axis of rotation, tilt control means for tilting the angle of a scan plane formed by said rotation relative to the axis of translation to an angle other than 90xc2x0, scan control means for collecting data by a helical scan employing said multi-row detector with the scan plane tilted, and image producing means for producing an image based on the collected data, characterized in that said X-ray CT apparatus further comprises: preprocessing means for applying preprocessing such as sensitivity correction to said data; tilt correcting processing means for applying tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane; multi-slice/helical interpolation processing means for applying multi-slice/helical interpolation processing for calculating interpolated data from proximate data in an image reconstruction plane; and backprojection processing means for applying backprojection processing to the interpolated data to produce an image.
The X-ray CT apparatus of the tenth aspect can suitably implement the image producing method of the second aspect.
In accordance with an eleventh aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that said tilt correcting processing means comprises: data position shifting means for shifting the positions of data arranged in a two-dimensional array along a channel index axis and a view index axis so that the view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane is canceled out; data extracting means for extracting a range of data in which data are completely present for all the views in the view direction from the shifted data array; dummy data appending means for appending the extracted data with dummy data to adjust the data range; and data transformation means for transforming the data into data enabling alignment of the channel positions through all views.
The X-ray CT apparatus of the eleventh aspect can suitably implement the image producing method of the third aspect.
In accordance with a twelfth aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that: said data position shifting means shifts the positions of parallelized data of the channels in a j-th (is the detector row index and 1xe2x89xa6jxe2x89xa6J) detector row by:
jxe2x80x94deltxe2x80x94iso=Ljxc2x7tan xcex8xc2x7sin{2xcfx80(pvnxe2x88x921)/VWN}
in the channel direction, where pvn is the view index and 1xe2x89xa6pvnxe2x89xa6VWN, rotation is made substantially for 2xcfx80 for all views, the tilt angle is represented as xcex8, and a distance from an intersection of the axis of translation and the axis of rotation to a scan plane corresponding to the j-th detector row is represented as Lj.
The X-ray CT apparatus of the twelfth aspect can suitably implement the image producing method of the fourth aspect.
In accordance with a thirteenth aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that: said data extracting means extracts data from a (Roundup{Ljxc2x7tan xcex8/DMM}+1+j_delt_iso )-th channel to a (Ixe2x88x92Roundup{Ljxc2x7tan xcex8/DMM}xe2x88x921+j_delt_iso)-th channel in the j-th detector row for a pvn-th view, where DMM is the channel-to-channel distance, and Roundup{ } is a roundup function.
The X-ray CT apparatus of the thirteenth aspect can suitably implement the image producing method of the fifth aspect.
In accordance with a fourteenth aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that said tilt correcting means comprises: data extracting means for extracting data from a (Roundup{Ljxc2x7tan xcex8/DMM}+1+j_delt_iso)-th channel to a (Ixe2x88x92Roundup{Ljxc2x7tan xcex8/DMM}xe2x88x921+j_delt_iso)-th channel in a j-th (j is the detector row index and 1xe2x89xa6jxe2x89xa6J) detector row for a pvn-th view, where pvn is the view index and 1xe2x89xa6pvnxe2x89xa6VWN, rotation is made substantially for 2xcfx80 for all views, the tilt angle is represented as xcex8, a distance from an intersection of the axis of translation and the axis of rotation to a scan plane corresponding to the j-th detector row is represented as Lj, DMM is the channel-to-channel distance, Roundup{ } is a roundup function, and
jxe2x80x94deltxe2x80x94iso=Ljxc2x7tan xcex8xc2x7sin{2xcfx80(pvnxe2x88x921)/VWN};
dummy data appending means for appending the extracted data with dummy data to adjust the data range; and data transformation means for transforming the data into data enabling alignment of the channel positions through all views.
The X-ray CT apparatus of the fourteenth aspect can suitably implement the image producing method of the sixth aspect.
In accordance with a fifteenth aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that said dummy data are air data.
The X-ray CT apparatus of the fifteenth aspect can suitably implement the image producing method of the seventh aspect.
In accordance with a sixteenth aspect, the present invention provides the X-ray CT apparatus having the aforementioned configuration, characterized in that said data transforming means is interpolating means.
The X-ray CT apparatus of the sixteenth aspect can suitably implement the image producing method of the eighth aspect.
According to the image producing method and X-ray CT apparatus, when a helical scan is conducted employing a multi-row detector with a scan plane tilted, an image can be obtained with artifacts suppressed. Thus, a tilted scan can be performed circumventing portions that are desirably kept out of radiation exposure (e.g., a fetus) or portions that cause artifacts when irradiated with X-rays (e.g., a portion embedded with a metal).
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.